A gas meter that uses a thermal flow sensor has been proposed wherein the speed of flow is increased at the position of a wall surface by partitioning, into a plurality of parallel minute flow paths, in the direction of flow of a fluid, the internal portion of a flow path wherein the a flow rate sensor has been attached to the wall surface, and a nozzle portion is provided in the middle of the flow path, to thereby increase the speed of flow at the nozzle portion, to thereby increase the detection sensitivity and to detect minute flow rates. Such a gas meter has been described, for example in Japanese Unexamined Patent Application Publication H4-69521and Japanese Unexamined Patent Application Publication H11-173896, each of which is incorporated by reference in its entirety herein.
However, when the detection sensitivity is increased for a minute flow, through the means described above, the detection sensitivity is also increased for a large flow rate commensurately, saturating the output of the flow rate sensor when there is a large flow rate, and thus there is a problem in that the measurement is not possible when there is a large flow rate. For this reason, it is difficult to detect accurately down to a minute leakage flow rate of about 5 liters per hour (l/h) in a gas meter for, for example, a maximum measurable flow rate of 30,000 l/h.
On the other hand, in order to increase the detection accuracy it is important to prevent the adhesion, to the flow rate sensor, of contaminants (dust) that are mixed into the gas to be measured. Given this, various attempts have been made in the past to interpose dust-removing filters into the flow path of the gas to be measured. However, in low-pressure gas meters wherein the supply pressure of the gas to be measured is about 2 Kilopascals (kPa), the allowable pressure loss is about 200 Pascals (Pa) at the most, and thus there are problems in providing a dust-removing filter wherein there is a large pressure loss. In particular, in a so-called small-mesh dust-removing filter wherein the dust capture ratio is high, the pressure loss is large, and thus it is still difficult to incorporate these into a low-pressure gas meter as described above.